Republican candidates have mostly focused on bashing Clinton on the stump, almost ignoring Sanders, but he's drawn increased attention from the GOP as he rises in the early states.

But Rubio did not hold his fire on Clinton, repeating his belief that Clinton is "disqualified' thanks to her use of a private email server and that she "lied" to the families of the victims of the 2012 Benghazi terrorist attack, a line that's become one of his favorite from the stump.

And he framed the Democrats in general as the cause of the "overtaxation problem in America."

"Their solution to everything is taxes. Concussions in football? Let's put a tax on it," he joked.

"You want to know why businesses are leaving America? We have one of the most tax-crazy environments in the world."

"The most important thing the federal government does is supposed to keep us safe. The world is a dangerous place ... [but] in the face of this growing danger, they are gutting our military," Rubio said of the Obama administration, before pivoting towards unnamed GOP rivals.

"I find it unacceptable that people running as Republicans who have voted to spend even less."

It was a seeming shot at Cruz, whom Rubio has accused of voting for a budget that cuts military spending. Cruz has brushed that aside, noting that he's voted for an amendment specifically offered by Rubio to raise military spending.

During a question-and-answer session, Rubio also pushed back at the onslaught of negative advertising he's faced from both Republicans and the Democrats.

"There have been $25 million spent in attacks against me. That didn’t come from $20 checks, those were multimillion-dollar checks raised to attack," he said, noting that he's been told to wait his turn and that he's too young to run for president.

"Every time we listen to the people in Washington who say this is who we should nominate, we lose," he added.